Silicone rubber compositions are traditionally used in the encapsulation and sealing of electric and electronic parts for the purpose of preventing or retarding corrosion and degradation thereof. When electric and electronic parts are exposed to sulfur-containing gases such as sulfur gas and sulfur dioxide gas, conventional silicone rubbers fail to prevent or retard the sulfur-containing gas from reaching the electric and electronic parts. In particular, they are not effective for preventing or retarding corrosion of metal parts.
As modern devices comprising precision electronic parts are being reduced in size and power, challenges are made to a transition from copper to silver for the material of electrodes and chip resistors, and a narrowing of the gap between electrodes. Thus electronic parts become more susceptible to corrosion with sulfur-containing gas. JP-A 2003-096301 (US 2003/0105206 A1) discloses a silicone rubber composition for use in the encapsulation and sealing of electric or electronic parts which is obtained by adding 0.5 to 90% by weight of a metal powder which is sulfidable with sulfur-containing gas to an organopolysiloxane compound. The composition is effective for preventing or retarding the sulfur-containing gas from reaching the electric or electronic parts. The organopolysiloxane composition with such high loadings of metal powder, however, has the tendency that metal particles agglomerate locally, with the probable risks of short-circuiting between electrodes and reducing insulation resistance. When the composition is stored for a long term, the metal powder can settle down due to a higher specific gravity than the organopolysiloxane.